1. Field of the Invention
The invention pertains to the field of half-toning image data in electronic reproduction technology and relates to a method for generating a threshold value matrix which is used in the frequency-modulated half-toning of the image data to be reproduced. The image data describes the content of a printed page or of a printed sheet on which a plurality of printed pages are combined. The printed pages contain images, texts and graphic elements, whose image data has previously been assembled in an electronic system for the creation and processing of printed pages in accordance with a layout.
In multicolor printing, the image data to be reproduced is generally printed in the four printing inks cyan, magenta, yellow and black (CMYK). The originals of the images are scanned optoelectronically, point by point and line by line, for example in a color scanner, in order to obtain for each scanned image point the color components red, green and blue (RGB) as color values. The color values of a scanned colored image are then converted by a color correction calculation into the color separation values for the color separations cyan, magenta, yellow and black. Following the conversion, for each image point four color separation values (CMYK) are available as tonal values in the value range from 0 to 100%. The color separation values are a measure of the ink densities with which the four printing inks cyan, magenta, yellow and black are printed on the printing material. In special cases, in which printing is carried out with more than four printing inks (decorative inks), each image point is identified by as many color separation values as there are printing inks. The color separation values can be encoded digitally, for example with 8 bits for each image point and printing ink, the value range from 0 to 100% being subdivided into 256 tonal value steps.
In addition to image data for colored images, during the electronic production of printed pages, image data for text and for graphic elements which, together with the images, are combined in accordance with the stipulations of a layout to form the image data for entire printed pages, is produced. The data from a plurality of printed pages is finally mounted to form the image data for a printed sheet. The printed sheet data is likewise provided as color separation values (CMYK) for output or intermediate storage.
Different tonal values of a color separation to be reproduced may be reproduced in the print only by an area modulation of the printing inks applied, that is to say by half-toning. Therefore, with the aid of the color separation values (CMYK), four half-toned color separations for the printing ink cyan, magenta, yellow and black are exposed in a color separation exposer, also called a recorder or imagesetter, point by point and line by line, on a recording material. The recording material can be a lithographic film, with which printing plates are subsequently produced in a recopying method, or the printing plates are exposed directly in the exposer. The printing plate exposer can also be integrated into a press. The half-toned color separations are used as printing forms for the multicolor printing. In the press, the overprinting of the differently inked half-toned printing forms is then carried out to form a multicolor reproduction. There are also digital presses which print the half-toned color separation data directly onto the printing material without the use of a printing plate, for example with an electrophotographic or inkjet printing process.
For the area modulation of the printing inks, methods for dot half-toning are known in which different tonal values of the color separation data are converted into half-tone dots of proportional size. The half-tone dots are disposed in half-tone cells, into which the area of the image data is subdivided regularly in two orthogonal directions. The half-tone cells are very small, for example there are square half-tone cells with a side length (screen width) of 1/60 cm, so that the printed half-tone dots are integrated again by the eye when viewing the final printed product, to form continuous tonal values. As a result of the overprinting of the periodically arranged half-tone dots, disruptive Moiré structures can occur in the print. In order to minimize such structures, the dot matrices of the four printing inks are arranged at different screen angles, for example at the screen angles 0, 15, 45 and 75 degrees.
In the known methods for dot half-toning, the half-tone dot in a half-tone cell is generally produced by a threshold value matrix. The recording area is resolved into the exposure points which are an order of magnitude smaller than the half-tone dots. The threshold value matrix contains the area of one or more half-tone dot cells and contains a threshold value for each exposure point within this area. During the recording of the color separations, the half-tone dots in the individual half-tone cells are assembled from exposure points. The decision as to whether an exposure point, as part of a half-tone dot within a half-tone cell, is to be exposed or not is made by comparing the tonal values of the color separation data with the threshold values of the threshold value matrix at the location of the respective exposure point, by which the tonal values are converted into high-resolution binary values having only two lightness values (exposed or not exposed), which form the pattern of the modulated dot matrix.
By using the known dot matrix methods, half-tone systems with any desired screen widths and screen angles and very good reproduction quality can be produced. However, in practice it has proven to be difficult to find screen systems for all possible applications which exhibit no Moiré. Such particular applications are given when more than four printing inks are to be overprinted or when the dot matrix with the fine patterns of the image content, for example a textile pattern, produces disruptive Moiré structures.
As an alternative to the dot matrix method, which is also designated amplitude-modulated half-toning, the area modulation of the printing inks can also be carried out in accordance with frequency-modulated half-toning (noise half-toning; stochastic half-toning), in which the tonal values of the color separation data are reproduced by an arrangement of small printed dots, generally of the same size, which are distributed in the recording area randomly but also as uniformly as possible. The number of printed dots per unit area determines the tonal value reproduced.
In many methods of frequency-modulated half-toning, the decision as to whether an exposure point is to be exposed or not is also brought about by comparing the tonal values of the image data to be reproduced with the threshold values of a threshold value matrix which contains a random distribution of the threshold values. For instance, the threshold value matrix is square and contains 256×256 threshold values. The threshold value matrix is repeated periodically horizontally and vertically, so that the entire recording area is covered without any gaps by threshold value matrices. Each exposure point in the recording area is also assigned a threshold value. During the half-toning, for each exposure point a tonal value of the color separation data is compared with the associated threshold value of the matrix. If the tonal value is greater than the threshold value, the exposure point is exposed; when the tonal value is less than or equal to the threshold value, the exposure point is not exposed. With the increase in the tonal value, the number of exposed points increases until, finally, for the maximum tonal value, the entire area covered by the threshold value matrix is exposed. In this way, for the entire printed page or the entire printed sheet, a random distribution of the exposed points is produced. On the basis of the random distribution of the exposed points, it is not possible for any Moiré to be produced by regular patterns of the image content. If a different distribution of the threshold values in the threshold value matrix is selected for each of the color separations CMYK, no Moiré structures are produced by the overprinting of the color separations either.
In order to produce a suitable distribution of the threshold values within the threshold value matrix, various methods are known. Here, the intention is to produce, first, a random distribution of the exposure points for all tonal values of the image data but, second, a distribution of the exposure points which does not contain any disruptive patterns and which appears uniform enough to the eye.
In U.S. Pat. No. 5,111,310, a method is described for producing a threshold value matrix whose threshold value distribution has the spectrum of blue noise, as it is known, that is to say the spectrum has no components at low frequencies below a limiting frequency. This has the effect that the threshold value distribution for all tonal values produces visually pleasant patterns for exposure points which do not act so unsteadily as a distribution with white noise. The distribution of the threshold value is constructed by an iterative method which, for each tonal value, transforms the pattern of the exposure points produced into a spectral representation, shapes the spectrum with a filter in accordance with the spectrum of the blue noise and then transforms it back into a modified exposure pattern. From the comparison between the modified exposure pattern and the original exposure pattern, a decision is made as to which threshold values of the threshold value matrix must be modified in order to come closer to the intended blue noise.
U.S. Pat. No. 5,579,457 discloses a method for producing a threshold value matrix in which the matrix is subdivided into subregions and, in each subregion, the threshold values are put into the matrix in accordance with a randomly placed spiral function. As a result, patterns of the exposure points which are arranged along randomly distributed spiral arms are produced. With increasing tonal value, further exposed points are added, which fill up the gaps in the spiral arms, that is to say the order in which the exposure pattern is supplemented by further exposed points with increasing tonal value is determined by the spiral functions. In this way, randomly distributed, rounded shapes are produced, which act uniformly and steadily on the eye.
The known methods of frequency modulated half-toning occasionally have a certain residual unsteadiness in the printed image, particularly in the average tonal values, which are felt to be disruptive. A further problem is that, in specific tonal value ranges, the exposure pattern produced contains too many exposure points which stand isolated, which can be exposed only poorly on the printing plate because of their small size and then cannot be transferred reliably to the printing material in the press.